Image manipulation programs are used to modify or otherwise use image content. For example, an image manipulation program can generate an image mask to at least partially isolate a portion of an image such that the image portion can be positioned on top of a different background image. For example, an image mask can be used to isolate a person from a group of individuals depicted in a first image, such as a picture of a wedding, and place that person in a different image, such as a picture of a famous landmark. The mask can include an outer boundary that generally tracks the outline of the object to be isolated. The mask can identify a region of the image portion that is opaque, thereby preventing the background image from being viewed through the opaque image. The mask can also identify a feathered region of the image portion that is at least partially transparent, thereby allowing at least a portion of the background image to be viewed through feathered region. The feathered region can also specify that the amount of transparency gradually increases between the opaque region of the image portion and an edge of the mask.
Existing solutions for providing a variable amount of transparency in a feathered region generate multiple curves that are outwardly offset from an opaque portion in an image mask. These solutions interpolate an amount of transparency between the curves. These solutions can generate loops and other unexpected curve shapes, thereby causing retrograde regions in the feathered region that fail to generally follow the outline of the object to be isolated using the image mask. The presence of retrograde regions in a feathered region can decrease the aesthetic quality of the feathered region of an image mask.